As a technique for inhibiting gene expression, RNA interference (RNAi) is known, for example (Non-Patent Document 1). Inhibition of gene expression by RNA interference generally is carried out by administering a short double-stranded RNA molecule to a cell or the like, for example. The double-stranded RNA molecule generally is called siRNA (small interfering RNA). It has been reported that not only siRNA but also a circular RNA molecule that is rendered partially double-stranded by intermolecular annealing also can inhibit gene expression (Patent Document 1). However, the RNA molecules used in these techniques to induce the inhibition of the gene expression have the following problems.
First, in order to produce the siRNA, it is necessary to synthesize a sense strand and an antisense strand separately and to hybridize these strands at the end of the process. Thus, there is a problem of low manufacturing efficiency. Furthermore, when the siRNA is administered to a cell, it is necessary to administer the siRNA to the cell while inhibiting the dissociation to single-stranded RNAs, which requires a laborious task of setting the conditions for handling the siRNA. On the other hand, the circular RNA molecule has a problem in that its synthesis is difficult.
These RNA molecules basically are composed of nucleotide residues. At present, in order to impart some function to the RNA molecules or to label the RNA molecules, there is no other way but to modify any of the components, i.e., a base, a sugar residue, or a phosphate group, of the nucleotide residue(s). Therefore, in the development of pharmaceuticals and the like utilizing RNA interference, it is very difficult to alter the RNA molecules so as to impart a further function thereto or to label them while maintaining their function of inhibiting the gene expression.